What is feminism, and does it collide with freedom of sexual expression in public?

There are two parts to your question.

Feminism is hard to define because it means different things to different people. At one extreme you have the Femen who are hard-core protesters who use shock-tactics to try to get what they want. At the other end you have the part-time feminists who fly the flag only when it suits them. Perhaps the most effective feminists are those placed at different positions along that very long line who work determinedly to improve women's rights.

I remember how badly women were treated in the 1940s and 1950s. They were considered to be second-rate citizens and, in many ways, inferior to men. Maybe characters like "Rosie the Riveter" helped to draw attention to the hypocrisy of the situation but there was a screaming need for somebody to stand up and do something. The impetus began in the 1970s and although there's still some distance to travel, women have come a long way. 

Not all women want to be feminists and I've hard angry protests from some. Yet those same women would not turn their backs on the advances feminism has made.

The second part: does feminism collide with sexual expression in public? I don't think so. Good manners determine how much sexual expression is appropriate in public.

P.S. I'd like to express my appreciation to Randy D for lending me those two tildes I used for a break. 

What about those feminist women in New York, who have won the legal right to go around topless, because the men can. They see it as having equal rights to men, rather than a form of sexual expression.

Your definitions don't count because a lot of people don't care what the definition is. They will die shouting some slogan and they don't even know what they are saying.

It's religious: the whole country is possessed by a spirit of bonkers.

Given than men can go bare chested without raising an eyebrow, Emma could have expressed feminism by going topless entirely. From what I recall, "bra burning" was considered the height of feminist expression once upon a time. Free The Nipple is a feminist organisation.
It's a storm in a teacup, driven by a flat-chested shrew who equates feminism with misandry. Germaine Greer did feminism no favours, she's not a feminist, she's a femocrat who doesn't believe that men should have the right to vote. Or possess testicles.

Actually I saw the attack as a rampant case of tall poppy syndrome. "Here are my tits, it annoys me because I don't have any." Do feminists have to be unattractive? Is it anti-feminist to embrace one's body? The only thing that could have inspired this bitter, unwarranted rant is the green-eyed monster. Envy, sticking out in lumps.
Had Ms Watson recently given birth and breastfed her child in public, it's highly likely that the same attacks would have surfaced. Again driven by jealousy. The world has GOT to stop sexualising breasts!

I support the literal definition of feminism that you find in a dictionary. I run into some conflicts with what a select few people ( both men and women, for and against) choose to define it as in their heads.   That's true for many movements.

Feminism has nothing to do with sexuality or sexual expression IMO.  It does have to do with equal rights under the law, equal pay, equal opportunity.  What bothers me the most about Hartley-Brewer's tweet is that she felt the need to belittle and criticize another woman in a very spiteful and hateful way over something she KNOWS is a problem within her industry.  For the life of me I don't understand women who feel the need to attack other women rather than support them.  The only thing I can come up with is jealousy and/or the need for attention.